The present invention relates to a process for upgrading hydrocarbon mixtures such as gasolines. More particularly, the invention relates to a process for decreasing the quantity of potentially harmful substances, such as benzene, in a hydrocarbon mixture while increasing the octane rating of the mixture.
In the petroleum industry, environmental and health considerations are leading to changes in processes for refining and/or reformulating gasoline. The use of leaded compounds in gasoline in order to boost its octane rating has been substantially discontinued, and economical alternatives for boosting the octane rating of gasoline are therefore needed. Furthermore, newly emerging regulations relating to automobile fuel emissions have prompted efforts to develop an economical process for reducing the content of benzene in gasoline.
In conventional petroleum processing, catalytically reformed naphtha is a major component of the total gasoline pool. Catalytically reformed naphtha also is the primary source of benzene in automotive fuels. Thus, a reduction in the benzene content of naptha reformate would contribute substantially to a reduction in the benzene content of a gasoline blend.
A variety of chemical reactions occur in the reforming of naphtha. These include dehydroaromatization of naphthenes, dehydrocyclization of paraffins, isomerization of paraffins, hydrocracking of paraffins and naphthenes, and hydrodealkylation of aromatics. The drawback of such processes is that they result in the production of a substantial quantity of n-paraffins, thereby lowering the octane rating of the reformate. The ratio of octane-reducing n-paraffins to octane-boosting i-paraffins in the reformate depends upon equilibrium at the reaction temperature. An increase in reforming temperature leads to an increased amount of n-paraffins.
Conventional processes for upgrading gasolines attempt to separately solve the problems of the presence of benzene and the presence of n-paraffins in reformate. In general, known processes for decreasing the amount of n-paraffins in gasoline-type feedstocks involve either isomerization of n-paraffins at moderate temperatures to shift equilibrium, or selective cracking of n-paraffins into liquefied petroleum gas (LPG) components at temperatures over 300.degree.-400.degree. C., particularly over zeolite A or erionite-type materials. The narrow pores of these zeolites do not allow any molecules other than n-paraffins to enter, and thus only n-paraffins are subject to cracking using such zeolites. Furthermore, these methods result in a decrease in the overall yield of gasoline product, because a portion of the feed is converted to LPG compounds.
Several techniques have been suggested for lowering the benzene concentration of reformate. These techniques generally involve either changing reforming feed or conditions, extracting aromatics, or alkylating aromatics with olefins. When the reforming feed or conditions are changed, a significant loss in the amount of feedstocks available for gasoline blending usually results. Extraction of the aromatics has limited economic viability, and is useful only in plants in which benzene can be used or from which it can be sold in the marketplace. Alkylation of aromatics has been conducted in various ways, including by using zeolite catalysts, as disclosed in U.S. Pat. Nos. 5,087,784 and 2,904,607. Another method which is known for alkylating aromatics involves predistillation of the reformate, followed by alkylation of its light fraction containing benzene with the addition of olefins, particularly propylene, over a fixed bed of solid phosphoric acid catalyst. Following alkylation, the light fraction is stabilized and then blended with the resulting product. Yet another alkylation method, which is disclosed in French Patent 2,640,994, involves fractionating reformate and contacting it with olefins over mordenite-type zeolite in order to reduce the benzene content of the reformate. However, in at least the latter two methods for alkylating aromatics, the rate of conversion of benzene to alkylbenzene is generally quite low. Furthermore, none of the known techniques for benzene alkylation also provide for isomerization of n-paraffins in order to further increase the octane rating of the reformate.